A telescopic gun sight or a riflescope is a device that provides an accurate point of aim for firearms such as rifles, handguns and shotguns. It is used with other types of weapons such as bows and crossbows as well. A telescopic sight can significantly improve the functionality of a firearm by providing the shooter with a simple yet highly accurate means for aiming at distant targets.
FIG. 1 shows a common optical layout for riflescopes. With reference to this figure, a riflescope is comprised of an objective lens 1 which forms a first image of the target on the objective focal plane 2. The first image produced by the objective lens is upside down and laterally reversed. An image-erecting means comprising a pair of lenses 3a and 3b relays this first image to the eyepiece focal plane 4 and forms a second, upright and laterally correct image there. An eyepiece lens 5 takes this second image and produces a virtual magnified image for the shooter to see.
Most modern riflescopes sold on the market today have variable magnification or “zoom” capability. With reference to FIG. 1, the magnification of the telescopic sight can be changed by changing the position of the relay lenses 3a and 3b. In this case, these lenses are commonly referred to as the “variator group”. Most riflescopes used for hunting and sporting applications have a zoom ratio of 3× or 4×, although higher zoom ratios up to 8× have also being introduced.
In riflescopes, the “point of aim” is usually designated by a reticle or cross hairs. In FIG. 1, the reticle 20 is mounted coplanar with the eyepiece focal plane 4. However, it can also be mounted at the objective focal plane 2. In either case, the shooter will see an image of the reticle superimposed on the image of the target.
Telescopic sights are equipped with two control knobs for elevation (up-down) and windage (left-right) adjustments. These knobs mechanically move the reticle so that the sight's point of aim can be aligned with the firearm's “point of impact”. The amount of adjustment is usually very small and is measured in minutes of angle (MOA) or milliradians. The adjustment mechanism is shown in FIG. 2. With reference to this figure, the image-erecting lenses 3a and 3b are mounted inside an inner tube 6. The rear end of this inner tube is attached to the main riflescope housing 30 via a hinge 37. This hinge is such that the inner tube 6 can tilt in both vertical and horizontal directions (FIG. 2 is a side-view so it only shows vertical tilt). The front end of the inner tube 6 is held in position by an adjustable knob 38 and a support leaf spring 9. The shooter can adjust the apparent position of the reticle 20 on the target image by adjusting the knob 38. The advantage of this mechanism is that the shooter always sees the reticle at the center of his field of view no matter how much the point of aim is shifted to adjust for windage and elevation.
To aim at targets located at various distances, the shooter must compensate for bullet drop. This is due to the fact that the bullet will begin to respond to the effects of gravity the instant it is free from the mechanical constraints of the bore. Bullet drop is defined as the vertical distance of the projectile below the line of departure from the bore. For hitting a distant target an appropriate positive elevation angle is required that is achieved by angling the line of sight from the shooter's eye through the centerline of the telescopic sight downward toward the line of departure. This can be accomplished by adjusting the sight's elevation knob. However, this is not an easy task to do in the field due to several reasons:                The elevation adjustment knobs often provide adjustment values at very small steps, usually 0.25 MOA or 0.1 miliradian per click. This makes it difficult to adjust for targets at long distances since many clicks would be required to compensate for bullet trajectory.        The mechanical elevation adjustment mechanism described above utilizes the tension of a spring 9 to hold the erector lens tube 6. As a result, repeated elevation adjustments can cause a shift in the sight's original point of aim.        
The present application teaches a novel method for bullet drop compensation in zoom telescopic gun sights. The telescopic sight is designed such that the point of aim can be adjusted simply by adjusting the zoom knob. A key advantage of this invention is that the shooter does not need to make two adjustments to his scope (zoom and elevation) to shoot at a distant target. As he naturally increases the magnification (zoom), the scope automatically adjusts its point of aim for a longer distance. Other benefits and advantages of the invention are presented in Section VI-D.